I take it you know there is a whopping ledge that runs along the North Wall at 2/3 height - you can wander along this ledge and access the ab points (usually the one above the Tap) with ease.
If you are continuing to the top of the wall there are a few agreed lower off points up there to return to the main ledge, but the walk off access (which runs through private property) is no longer permissible. Some routes on the upper wall are no longer feasible because of this - Rick Sewards (the BMC access rep for the Wye Valley) who posts on here will be able to give you the best information, try emailing him through his profile if you have queries.
The best climbing on North Wall is all below the main ledge anyway.

As the Ivanator said, just the two on the Great Ledge, above Joe's Route and The Tap. There is also an ab station on the Main Ledge (the lower ledge above Simplex et al) which is useful to know about if you're abseiling off the The Tap (a full 50 metres) and you're worried your ropes are a bit short. On the far left-hand end (North Buttress), Yesterday's Dreams and the routes to its left have their own bolted ab stations/lower offs as detailed in the guide, and on the far right hand end Psychotic and the routes to its right also have bolted ab stations

Above the Great Ledge, there is an abseil station above the common finish to Bacchanalian, Finishing Under a Blood Red Sky and Exodus, and another one above the top pitch of Right Hand Route. These are (fairly recent) rope and ring abseil stations around trees rather than bolts - they should be fine but check before using them. Please don't attempt any of the other routes/finishes above the Great Ledge, as there will not be an ab point and we don't have permission any longer to exit through the garden.